I began reading a new novel which I see has a line height of 1.36 & that seems to be a comfortable spacing - better than the default of, I think 1.2 that most books display as.

So I wondered if there are any studies of e-book line heights & readability. google found this from an ebook designer's site:A Global line height of around 1.4 is ideal for screen readability. Studies indicate that the precise value varies between 1.3 and 1.6. Vook's default is 1.375.

it does not reference the "studies" though. does anyone have any useful links or even just subjective views ?

I also observed that the line-height is simply 1.36 not 1.36em - do all CSS numbers default to em if not explicitly stated ?

I don't know about any studies, but I know those numbers mentioned are all too big for me. I've settled on 1.1 for my reading (which is probably quite dependent on hardware, reading software, and font, truth be told). Most stuff 1.2 or bigger just looks goofy to me. I must be an outlier.

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I began reading a new novel which I see has a line height of 1.36 & that seems to be a comfortable spacing - better than the default of, I think 1.2 that most books display as.

Ideally, the books themselves would define NO line-height setting, so that the reader could set their preference consistently across all books with one setting in their reading app/device. I know if I decide to open the hood on a commercial epub, I always go ahead and remove any line-height css/styles that might apply to the normal body text of the book while I'm in there.

i think if no line heights are specified the Sony reader goes to 1em text and 1.2em line height.

my assertion that 1.2 is the Sony default is based on a) reading it somewhere & b) noting no difference in the display if I delete any height 1.2 declarations

google found this for me, & other similar links support the idea that " height 1.2 = default"normal
Depends on the user agent. Desktop browsers (including Firefox) use a default value of roughly 1.2, depending on the element's font-family.

Even if there were some study about the optimal line separation, there's no way it can be applied to CSS properties, because the result is completely dependent on the font metrics. Not only on whether the font has a large or small x-height, ascenders, etc. but on some arbitrary values in the font file, which you can change at will, and will cause the same "line-height" value to be rendered differently.

I usually do not specify a line-height attribute in my CSS, letting the reader determine that with their eBook settings. One novel I am reading has wide spacing between the paragraphs which is driving me crazy.

Even if there were some study about the optimal line separation, there's no way it can be applied to CSS properties, because the result is completely dependent on the font metrics. Not only on whether the font has a large or small x-height, ascenders, etc. but on some arbitrary values in the font file, which you can change at will, and will cause the same "line-height" value to be rendered differently.

+1

Any discussion of line height, (real typographic term is leading) has to take in consideration the various fonts' x-height (height of lower case letters relative to the font's point size). The line height (leading) that leads to the most readability will vary based on the font's x-height. Other factors in the design of the glyphs and other font metrics, like letter spacing and kerning, also contribute.